1. Technical Field
The present invention relates to a push-pull type electrostatic ultrasonic transducer, particularly, an electrostatic ultrasonic transducer capable of generating usual sound pressure with lower energy, and thereby, reducing voltage (lowering power), an ultrasonic speaker using the same, a method of designing the electrostatic ultrasonic transducer, a method of reproducing sound signal by means of the electrostatic ultrasonic transducer, an apparatus for designing the electrostatic ultrasonic transducer, a program for designing the electrostatic ultrasonic transducer, a method of manufacturing a fixed electrode of the electrostatic ultrasonic transducer, an ultra directional acoustic system and a display device.
2. Related Art
An electrostatic ultrasonic transducer has been usually known as a wide band oscillation type ultrasonic transducer capable of generating high sound pressure over a high frequency band. FIG. 7 shows an example of a structure of a wide band oscillation type ultrasonic transducer. The electrostatic ultrasonic transducer in FIG. 7 is called “the pull type” since it operates only in a direction that an oscillation film is pulled to a fixed electrode side.
The electrostatic ultrasonic transducer shown in FIG. 7 uses a dielectric 131 (an insulator) such as PET (polyethylene terephthalate resin) having around 3 to 10 μm in thickness as an oscillator (an oscillation film). An upper electrode 132 formed as metallic foil such as aluminum is formed into one body with the dielectric 131 on an upper surface of the dielectric 131 in a process such as vapor deposition. A lower electrode 133 made of brass is provided so as to be in contact with a lower surface of the dielectric 131. The lower electrode 133 is connected to a lead 152 and fixed to a base plate 135 made of Bakelite or the like.
The upper electrode 132 is connected to a lead 153, which is connected to a direct current bias power source 150. Around 50 to 150 V of direct current bias voltage for adherence of the upper electrode is always applied to the upper electrode 132 from the direct current bias power source 150 so that the upper electrode 132 would adhere to a lower electrode 133 side. 151 denotes a signal source.
The dielectric 131, the upper electrode 132 and the base plate 135 are fastened together with metal rings 136, 137 and 138 and a mesh 139 by means of a case 130.
On a surface of the lower electrode 133 on the dielectric 131 side, formed are plural minute grooves, which are not uniform in shape and around tens to hundreds μm in size. The minute groove forms a gap between the lower electrode 133 and the dielectric 131. Accordingly, distribution of electrostatic capacity between the upper electrode 132 and the lower electrode 133 varies slightly. The surface of the lower electrode 133 is manually roughed by means of a rasp in order to form the random minute grooves. In an electrostatic ultrasonic transducer, forming numberless condensers different in size and depth of a gap as described above allows frequency characteristics to be in a wide band (refer to JP-A-2000-50387 and JP-A-2000-50392, for example).
As described above, the electrostatic ultrasonic transducer shown in FIG. 7 has been usually known as a wide band ultrasonic transducer (of the pull type) capable of generating comparatively high sound pressure over a wide band.
The maximum value of the sound pressure, however, is low a little such as 120 dB or less, for example. This is insufficient a little in sound pressure for using the electrostatic ultrasonic transducer as an ultrasonic speaker. In order to obtain a sufficient parametric effect in an ultrasonic speaker, required is 120 dB or more of ultrasonic sound pressure. The electrostatic ultrasonic transducer (of the pull type), however, is difficult to achieve the above numerical value. Accordingly, a ceramic piezoelectric element such as PZT or a high-polymer piezoelectric element such as PVDF has been mostly used as an ultrasonic generator. The piezoelectric element, however, has a sharp resonance point regardless of a material and is driven at a frequency of the resonance to be put to practical use as an ultrasonic speaker. This causes an extremely small range of the frequency capable of securing high sound pressure, that is, a narrow band.
In order to solve such a problem, it is conceivable to provide an electrostatic ultrasonic transducer shown in FIG. 1 to which a designing method in accordance with the invention is applied. A structure of the above is generally called a push-pull type. Details of the structure and an operation thereof are described later. The ultrasonic transducer shown in FIG. 1 can simultaneously satisfy both of a wide band characteristic and the high sound pressure, differently from the pull type electrostatic ultrasonic transducer.
In the push-pull type electrostatic ultrasonic transducer shown in FIG. 1, an important problem is particularly the height “t” of convexes of fixed electrodes 10A and 10B (the height of a step of a hole with the step). The height “t” of convexes of the fixed electrodes 10A and 10B (the height of a step of a hole with the step) is usually set with a little room empirically at 10 to 20 μm, for example. Setting the height “t” of the convex high as described above causes requirement of high driving alternating current voltage corresponding to the above and excessive consumption of energy and this causes a problem. Accordingly, required is to provide a method of quantitatively designing the optimum height “t” of the convex on the basis of values of desired sound pressure and driving frequency.
Quantitatively obtaining the optimum height “t” of the convex allows an efficient structure with which desired pressure can be obtained at low driving voltage to be put into practice. In other words, the equal sound pressure can be generated with lower energy, so that the electrostatic ultrasonic transducer can be practically reduced in voltage (lowered in power).
As described above, in the push-pull type electrostatic ultrasonic transducer shown in FIG. 1, required is to provide a method of quantitatively designing the height “t” of a convex of the fixed electrodes 10A and 10B (the height of a step of a hole with the step). Quantitatively obtaining the optimum height “t” of the convex allows an efficient structure with which desired pressure can be obtained at low driving voltage to be put into practice. This allows the equal sound pressure to be generated with lower energy. That is to say, the electrostatic ultrasonic transducer can be practically reduced in voltage (lowered in power).